A magnetic field-to-voltage converter using a magnetic MOSFET (MAGFET) device has been designed, simulated and tested. The resulting sensor was measured under magnetic fields ranging from 0 to 0.8 T, the obtained sensitivity was 0.03 {\rm T}^{-1} with an offset lower than 0.2%. SPICE macro model for the MAGFET in the saturation region is presented. Using an analogue Hardware Description Language (HDL-A) we have simulated the behaviour of the MAGFET and an interface circuit based on a custom current-mode sigma delta modulator.